Evidence for a modulatory role of orexin A on the nitrergic neurotransmission in the mouse gastric fundus

Evidence that resistin acts on the mechanical responses of the mouse gastric fundus

Resistin, among its several actions, has been reported to exert central anorexigenic effects in rodents. Some adipokines which centrally modulate food intake have also been reported to affect the activity of gastric smooth muscle, whose motor responses represent a source of peripheral signals implicated in the control of the hunger-satiety cycle through the gut-brain axis. On this basis, in the present experiments, we investigated whether resistin too could affect the mechanical responses in the mouse longitudinal gastric fundal strips. Electrical field stimulation (EFS) elicited tetrodotoxin- and atropine-sensitive contractile responses. Resistin reduced the amplitude of the EFS-induced contractile responses. This effect was no longer detected in the presence of L-NNA, a nitric oxide (NO) synthesis inhibitor. Resistin did not influence the direct muscular response to methacholine. In the presence of carbachol and guanethidine, EFS elicited inhibitory responses whose amplitude was increased by resistin. L-NNA abolished the inhibitory responses evoked by EFS, indicating their nitrergic nature. In the presence of L-NNA, resistin did not have any effect on the EFS-evoked inhibitory responses. Western blot and immunofluorescence analysis revealed a significant increase in neuronal nitric oxide synthase (nNOS) expression in neurons of the myenteric plexus following resistin exposure. In conclusion, the present results offer the first evidence that resistin acts on the gastric fundus, likely through a modulatory action on the nitrergic neurotransmission.

Nitric Oxide: From Gastric Motility to Gastric Dysmotility

It is known that nitric oxide (NO) plays a key physiological role in the control of gastrointestinal (GI) motor phenomena. In this respect, NO is considered as the main non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter responsible for smooth muscle relaxation. Moreover, many substances (including hormones) have been reported to modulate NO production leading to changes in motor responses, further underlying the importance of this molecule in the control of GI motility. An impaired NO production/release has indeed been reported to be implicated in some GI dysmotility. In this article we wanted to focus on the influence of NO on gastric motility by summarizing knowledge regarding its role in both physiological and pathological conditions. The main role of NO on regulating gastric smooth muscle motor responses, with particular reference to NO synthases expression and signaling pathways, is discussed. A deeper knowledge of nitrergic mechanisms is important for a better understanding of their involvement in gastric pathophysiological conditions of hypo- or hyper-motility states and for future therapeutic approaches. A possible role of substances which, by interfering with NO production, could prove useful in managing such motor disorders has been advanced.

Glucagon-like peptide-2 modulates the nitrergic neurotransmission in strips from the mouse gastric fundus

AIM

To investigate whether glucagon-like peptide-2 (GLP-2) influences the neurally-induced responses in gastric strips from mice, since no data are available.

METHODS

For functional experiments, gastric fundal strips were mounted in organ baths containing Krebs-Henseleit solution. Mechanical responses were recorded via force-displacement transducers, which were coupled to a polygraph for continuous recording of isometric tension. Electrical field stimulation (EFS) was applied via two platinum wire rings through which the preparation was threaded. The effects of GLP-2 (2 and 20 nmol/L) were evaluated on the neurally-induced contractile and relaxant responses elicited by EFS. Neuronal nitric oxide synthase (nNOS) enzyme was evaluated by immunohistochemistry.

RESULTS

In the functional experiments, electrical field stimulation (EFS, 4-16 Hz) induced tetrodotoxin (TTX)-sensitive contractile responses, which were reduced in amplitude by GLP-2 (P < 0.05). In the presence of the nitric oxide (NO) synthesis inhibitor L-NNA, GLP-2 no longer influenced the neurally-evoked contractile responses (P > 0.05). The direct smooth muscle response to methacholine was not influenced by GLP-2 (P > 0.05). In the presence of guanethidine and carbachol, the addition of GLP-2 to the bath medium evoked TTX-sensitive relaxant responses that were unaffected by L-NNA (P > 0.05). EFS induced a fast NO-mediated relaxation, whose amplitude was enhanced in the presence of the hormone (P < 0.05). Immunohistochemical experiments showed a significant increase (P < 0.05) in nNOS immunoreactivity in the nerve structures after GLP-2 exposure.

CONCLUSION

The results demonstrate that in gastric fundal strips, GLP-2 influences the amplitude of neurally-induced responses through the modulation of the nitrergic neurotransmission and increases nNOS expression.

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.

Influence of obestatin on the gastric longitudinal smooth muscle from mice: mechanical and electrophysiological studies

Obestatin is a hormone released from the stomach deriving from the same peptide precursor as ghrelin. It is known to act as an anorectic hormone decreasing food intake, but contrasting results have been reported about the effects of obestatin on gastrointestinal motility. The aim of the present study was to investigate whether this peptide may act on the gastric longitudinal smooth muscle by using a combined mechanical and electrophysiological approach. When fundal strips from mice were mounted in organ baths for isometric recording of the mechanical activity, obestatin caused a tetrodotoxin-insensitive decrease of the basal tension and a reduction in amplitude of the neurally induced cholinergic contractile responses, even in the presence of the nitric oxide synthesis inhibitor N(G)-nitro-l-arginine. Obestatin reduced the amplitude of the response to the ganglionic stimulating agent dimethylphenyl piperazinium iodide but did not influence that to methacholine. In nonadrenergic, noncholinergic conditions, obestatin still decreased the basal tension of the preparations without influencing the neurally induced relaxant responses. For comparison, in circular fundal strips, obestatin had no effects. Notably, in the longitudinal antral ones, obestatin only caused a decrease of the basal tension. Electrophysiological experiments, performed by a single microelectrode inserted in a gastric longitudinal smooth muscle cell, showed that obestatin had similar effects in fundal and antral preparations: it decreased the resting specific membrane conductance, inhibited Ca(2+) currents, and positively shifted their voltage threshold of activation. In conclusion, the present results indicate that obestatin influences gastric smooth muscle exerting site-specific effects.

Endocannabinoids modulate non-adrenergic, non-cholinergic inhibitory neurotransmission in strips from the mouse gastric fundus

AIM

To investigate the effects of endocannabinoids on non-adrenergic, non-cholinergic (NANC) relaxant responses in gastric strips from mice.

METHODS

Gastric longitudinal strips from the fundus region were mounted in organ baths for isometric recording.

RESULTS

In carbachol-precontracted strips, electrical field stimulation (EFS) elicited tetrodotoxin (TTX)-sensitive fast nitrergic relaxant responses that were followed, at the highest stimulation frequency, by sustained relaxations. The latter were abolished by α-chymotrypsin. Anandamide caused a TTX-sensitive relaxation that was abolished by α-chymotrypsin but unaffected by the nitric oxide (NO) synthesis inhibitor, Nω-nitro-L-arginine (L-NNA). Anandamide reduced the amplitude of EFS-induced fast relaxations, whereas increased that of sustained ones. Relaxation to the nicotinic receptor agonist dimethylphenyl piperazinium iodide (DMPP) was decreased in amplitude by either anandamide or L-NNA, whereas, surprisingly, it was increased by α-chymotrypsin and abolished by L-NNA plus α-chymotrypsin. Relaxation to vasoactive intestinal polypeptide (VIP) was not influenced by anandamide or L-NNA and was abolished by α-chymotrypsin. Following VIP desensitization, fast relaxant responses to EFS were reduced and the sustained ones abolished. The CB1 receptor antagonist AM251 increased, only at the highest stimulation frequency, the amplitude of the EFS-induced fast relaxation and reduced the sustained one. AM251 increased the response to DMPP and abolished that to anandamide. The CB2 receptor antagonist AM630 had no effects.

CONCLUSION

These results indicate that endocannabinoids modulate, via prejunctional CB1 receptors, the NANC peptidergic neurotransmission that, in turn, affects the nitrergic one.

Orexins: tissue localization, functions, and its relation to insulin secretion and diabetes mellitus

Orexins play a role in many biological functions include sleep, feeding, and energy balance. They also regulate circadian rhythms and the way that we feel pain. Orexins have been identified in a variety of tissues including the cerebrospinal fluid, blood, hypothalamus, spinal cord, sensory ganglion, enteric nervous system, pituitary, adrenal, salivary and lacrimal glands, testis, vestibular gland, and skin. Orexins play a role in a variety of biological functions including arousal, sleeping, food and fluid intake, pain, memory, perception of odor, and sexual activity. Orexins have also been implicated in the regulation of glucose metabolism. The expression of orexin is induced by hypoglycemia, low food, pregnancy, and hemodialysis. In contrast, factors that inhibit the expression of orexins include obstructive sleep apnea, aging, depression, obesity, traumatic brain injury, and inflammatory molecules such as liposaccharide. In conclusion, orexins are widely distributed and involved in a large variety of biological activities.

Muscular effects of orexin A on the mouse duodenum: mechanical and electrophysiological studies

Orexin A (OXA) has been reported to influence gastrointestinal motility, acting at both central and peripheral neural levels. The aim of the present study was to evaluate whether OXA also exerts direct effects on the duodenal smooth muscle. The possible mechanism of action involved was investigated by employing a combined mechanical and electrophysiological approach. Duodenal segments were mounted in organ baths for isometric recording of the mechanical activity. Ionic channel activity was recorded in current- and voltage-clamp conditions by a single microelectrode inserted in a duodenal longitudinal muscle cell. In the duodenal preparations, OXA (0.3 μM) caused a TTX-insensitive transient contraction. Nifedipine (1 μM), as well as 2-aminoethyl diphenyl borate (10 μM), reduced the amplitude and shortened the duration of the response to OXA, which was abolished by Ni(2+) (50 μM) or TEA (1 mM). Electrophysiological studies in current-clamp conditions showed that OXA caused an early depolarization, which paralleled in time the contractile response, followed by a long-lasting depolarization. Such a depolarization was triggered by activation of receptor-operated Ca(2+) channels and enhanced by activation of T- and L-type Ca(2+) channels and store-operated Ca(2+) channels and by inhibition of K(+) channels. Experiments in voltage-clamp conditions demonstrated that OXA affects not only receptor-operated Ca(2+) channels, but also the maximal conductance and kinetics of activation and inactivation of Na(+), T- and L-type Ca(2+) voltage-gated channels. The results demonstrate, for the first time, that OXA exerts direct excitatory effects on the mouse duodenal smooth muscle. Finally, this work demonstrates new findings related to the expression and kinetics of the voltage-gated channel types, as well as store-operated Ca(2+) channels.

The hungry stomach: physiology, disease, and drug development opportunities

During hunger, a series of high-amplitude contractions of the stomach and small intestine (phase III), which form part of a cycle of quiescence and contractions (known as the migrating motor complex, MMC), play a "housekeeping" role prior to the next meal, and may contribute toward the development of hunger. Several gastrointestinal (GI) hormones are associated with phase III MMC activity, but currently the most prominent is motilin, thought to at least partly mediate phase III contractions of the gastric MMC. Additional GI endocrine and neuronal systems play even more powerful roles in the development of hunger. In particular, the ghrelin-precursor gene is proving to have a complex physiology, giving rise to three different products: ghrelin itself, which is formed from a post-translational modification of des-acyl-ghrelin, and obestatin. The receptors acted on by des-acyl-ghrelin and by obestatin are currently unknown but both these peptides seem able to exert actions which oppose that of ghrelin, either indirectly or directly. An increased understanding of the actions of these peptides is helping to unravel a number of different eating disorders and providing opportunities for the discovery of new drugs to regulate dysfunctional gastric behaviors and appetite. To date, ghrelin and motilin receptor agonists and antagonists have been described. The most advanced are compounds which activate the ghrelin and motilin receptors which are being progressed for disorders associated with gastric hypomotility.

Contribution of endogenous nitrergic and peptidergic influences to the altered neurally-induced gastric contractile responses in strips from dystrophic (mdx) mice

Gastrointestinal motor disorders have been reported to occur in dystrophic (mdx) mice. The aim of the present study was to investigate the contribution of endogenous nitrergic and peptidergic components to the gastric contractile responses in strips from wild type (WT) and mdx mice. In both preparations, electrical field stimulation (EFS) induced frequency-dependent excitatory responses that were abolished by atropine or tetrodotoxin. The amplitude of the neurally-induced contractile responses was greater in strips from mdx mice in respect to the WT ones. In both preparations, at the end of the stimulation period strip tension decayed below the pre-stimulus level (off-relaxations). The nitric oxide (NO) synthesis inhibitor L-NNA increased the amplitude of the EFS-induced contractile responses without influencing off-relaxations. alpha-chymotrypsin and PACAP 6-38 abolished off-relaxations and also caused a reduction in amplitude of the contractile responses, whereas VIP receptor antagonists were ineffective. The efficacy of L-NNA, alpha-chymotrypsin or PACAP 6-38 on the excitatory responses was lower in strips from mdx mice in respect to the WT ones. alpha-chymotrypsin, in the presence of L-NNA, was no longer able to decrease the amplitude of the neurally-induced contractile responses but still abolished off-relaxations in both preparations. Direct muscular responses to methacholine were similar in amplitude in the two preparations and were not influenced by L-NNA or alpha-chymotrypsin. The results indicate that both endogenous NO and peptides influence the EFS-induced cholinergic responses: a stronger peptidergic modulatory action on a weaker nitrergic neurotransmission is suggested to occur in strips from mdx mice in respect to the WT ones and to contribute to the altered gastric contractile responses.